Induction quenching may be employed as quench hardening on a bearing ring of a rolling bearing made of steel. This induction quenching has such advantages that equipment can be simplified and heat treatment in a short time is enabled, as compared with general quench hardening of heating the bearing ring in a furnace and thereafter dipping the same in a cooling liquid such as oil.
In order to simultaneously heat an annular region, to be quench-hardened, along a rolling contact surface of the bearing ring in the induction quenching, however, an induction heating member such as a coil for induction-heating the bearing ring must be arranged to be opposed to the rolling contact surface. In a case of quench-hardening a large-sized bearing ring, therefore, there are such problems that a large-sized coil responsive thereto and a power source of high capacity corresponding to this coil are required and the production cost for a quenching apparatus increases.
Transfer quenching employing a small-sized induction heating coil may be employed as a countermeasure for avoiding such problems. In this transfer quenching, high-frequency induction heating is executed with a coil arranged to be opposed to part of an annular region, to be heated, of a bearing ring for relatively moving along this region, and a cooling liquid such as water is injected toward the heated region immediately after passage of the coil thereby successively quench-hardening this region. In a case of merely employing this transfer quenching, however, a quench starting region and a quench ending region partially overlap with each other when the coil goes around from a region (quench starting region) where the quenching has been started and quench-hardens a region (quench ending region) to be finally subjected to the quenching. Therefore, occurrence of quench cracking resulting from re-quenching of the overlapping regions is apprehended. Further, regions adjacent to the aforementioned overlapping regions are heated to a temperature of not more than an A1 point and tempered following heating of the quench ending region, and hence there is also such an apprehension that hardness lowers. When the transfer quenching is employed, therefore, a countermeasure of leaving a region (soft zone) not subjected to quenching between the quench starting region and the quench ending region is generally employed. This soft zone has low yield strength and is also insufficient in abrasion resistance, due to low hardness. Therefore, in a case of forming a soft zone on a bearing ring, it is necessary to see to it that the soft zone does not become a load region.
On the other hand, there is proposed a method for executing the aforementioned transfer quenching forming a soft zone and thereafter cutting a region corresponding to the zone while fitting a stopper body subjected to quenching into this region (refer to Japanese Patent Laying-Open No. 6-17823 (Patent Document 1), for example). Thus, remaining of the soft zone having low hardness can be avoided.
There is also proposed a method for avoiding formation of a soft zone by employing two coils oppositely moving in the circumferential direction of a bearing ring (refer to Japanese Patent Laying-Open No. 6-200326 (Patent Document 2), for example). According to this method, occurrence of a re-quenched region can also be avoided while avoiding formation of a soft zone by starting quenching in a state where the two coils are arranged to be adjacent to each other and ending the quenching on a position where the same butt against each other again.